Polymerization reactions are important in forming various kinds of materials. Among the reactions, a radical polymerization reaction is widely used because the reaction proceeds fast and because various raw materials may be used in the reaction.
To be specific, in the radical polymerization reaction, a compound that generates radicals is added into a monomer or oligomer having a double bond such as an acrylate derivative, and a polymerization reaction is initiated and promoted. In many cases, the reaction proceeds so fast that heating for a long time is not required.
The fastness of the reaction is due to high reactivity of the radicals. On the other hand, the radicals have very short lives, and are easily deactivated by a deactivator such as oxygen. Thus, the reaction does not occur outside the region where the free radicals are generated.
Further, if the generated radicals are spatially inhomogeneous in amount, polymerized material is accordingly inhomogeneous in molecular weight, so that a highly reliable material cannot be obtained.
Especially, when a photocurable material containing a photoinitiator is radically polymerized by irradiation of light from the surface thereof and by consequent radical generation in order to provide a cured product having a certain thickness or larger, a deep portion of the material that the light is hard to reach is not sufficiently cured.
Thus, in order to cure the material with more sufficient homogeneity, larger amount of radicals are required to be generated. However, long-time photoirradiation and long-time heating are necessary to generate a large amount of radicals. The properties of the cured product may rather be deteriorated by the long-time irradiation and heating.
Further, if a method is provided by which a sufficiently cured product can be produced with less energies even in a portion that is easily irradiated with the light, the method is ideal from the viewpoint of workability and energy consumption.
In order to solve the problem described above, various kinds of sensitizers as additives have been studied (see Patent Literatures 1-4).